Receivers for optical signals are useful in a wide variety of applications, including optical communications systems. As presently contemplated, such systems typically have a light source and receiver optically coupled to each other by means of an optical fiber. Information is transmitted as variations in a characteristic, e.g., intensity, of the optical radiation, and the rate of variation in a digital transmission system is referred to as the bit rate. Other characteristics, such as frequency, of the optical radiation might be varied. The receiver detects the incoming optical signal and either regenerates the optical signal or converts it to an electrical signal.
Of course, receivers should have high sensitivity. They must, however, satisfy other requirements such as dynamic range. Additionally, typical optical communication systems may operate within a wide range of bit rates. A common approach to achieving high sensitivity and wide bandwidth for low bit rate, that is, several Mbit/sec, applications, is to use a transimpedance amplifier. This type of amplifier has a resistance to provide electrical feedback between the amplifier output and input, i.e., across the amplifier. In a typical configuration, the receiver uses a Si JFET amplifier and a P-I-N photodetector with less than 100 pA dark current. With this configuration, the feedback resistance must be greater than 500 megohms so that the Johnson noise from the resistance is less than the amplifier noise if it is assumed that the amplifier has a 1 MHz bandwidth.
As might be appreciated, such a large resistance causes several problems. For example, the dynamic range of the receiver is extremely limited because very large amplifier output voltage swings are needed for any appreciable current flow through the feedback resistance. For example, a 1 .mu.W input for the values assumed above will require a 500 volt swing. Additionally, the bandwidth is limited because any parasitic capacitance in parallel with the feedback resistance produces a very large RC time constant. It can be shown that for the assumed 1 MHz bandwidth, the parasitic capacitance must be less than 0.0003 pF.
In addition to their use in communications systems, optical receivers are used in many types of instruments such as optical time domain reflectometers. This type of instrument is used to analyze many optical fiber characteristics such as distances to splices, etc. Additionally, optical receivers are often used to detect the Cerenkov radiation from elementary particles.